Arimidex was the first aromatase inhibitor available.
It is therefore the most well known among anti aromatases and the one doctors have most experience with.

Note that anti aromatases (Aromasin, Femara, Arimidex) are often confused with anti estrogens (Nolvadex). Anti aromatases work quite differently than anti estrogens do. Anti estrogens are actually weak estrogens that attach to the estrogen receptor sites in the body (such as the nipple and breast area), by doing so they block the 'bad' estrogens from attaching to these sites. Anti aromatases on the other hand block the formation of estrogen in the body in the first place. They do this, by blocking the enzyme aromatase which is responsible for the conversion of testosterone to estrogen, so there is less estrogen circulating in the blood. In a sense, an anti aromatase such as Aromasin takes care of the problem right from it's root.